1. Field of the Invention
The present invention relates to flow sensors, and more particularly to a thermal dispersion flowmeter that compensates for variable mixed-gas compositions.
2. Discussion of the Related Art
Thermal dispersion flowmeters are a common choice for flow metering devices in the commercial and industrial metering markets. A typical sensor element for use in such meters is the resistance temperature detector (RTD), the resistance of which is related to the temperature of the element itself. A typical sensor employs at least two RTD elements. One RTD element is used as a reference element and is normally unheated. The second RTD is used as the active element which is heated. In use, the effect of flow on the heated RTD element provides a measure of the flow velocity of the substance in the duct or conduit being monitored.
Two different methods are commonly used in the thermal dispersion industry to determine the flow in a conduit. One technique is to maintain a constant temperature differential between the reference RTD and the active RTD. This method measures the voltage or current required to maintain the active RTD at a constant temperature above the reference RTD while heat is removed from the active RTD due to the physical properties of the flowing media. The other method measures the voltage difference between the active and the reference RTDs, while the active RTD is heated by a constant current or a constant power heat source. During this measurement, as with the other method, the active RTD loses heat by way of the physical properties of the flowing media.
One factor that affects the accuracy of conventional flowmeters is the consistency of the physical composition of flowing media. In many applications, the flowing media maintains the same general composition, in which case conventional flowmeters could provide sufficiently accurate flow rate measurements. However, in situations such as flare gases and other variable mixed-gas composition situations, the physical composition of the media is constantly changing, resulting in significant challenges for obtaining accurate flow rates.